Sperm are immunogenic - that is they will cause an autoimmune reaction if transplanted from the testis into a different part of the body. This has been demonstrated in experiments using rats by Lansteiner (1899) and Metchinikoff (1900),[11][12] mice [13] and guinea pigs.[14] The likely reason for this is that sperm first mature at puberty, after immune tolerance is established, therefore the body recognizes them as foreign and mounts an immune reaction against them. Therefore, mechanisms for their protection must exist in this organ to prevent any autoimmune reaction. The blood-testis barrier is likely to contribute to the survival of sperm. However, it is believed in the field of testicular immunology that the blood-testis barrier cannot account for all immune suppression in the testis, due to (1) its incompleteness at a region called the rete testis [12] and (2) the presence of immunogenic molecules outside the blood-testis barrier, on the surface of spermatogonia.[11][12] Another mechanism which is likely to protect sperm is the suppression of immune responses in the testis..http://en.wikipedia.org/wiki/Immune_privilege